Composition, bath, and process for chromium plating



Patented May 26, 1953 COMPOSITION, BATH, AND PROCESS FOR CHROMIUM PLATING Frank Passal', Detroit; Mich, as'signonto United Chromium, Incorporated, New York; N. Y.; a-

corporation of Delaware Application November 23, 1949, Serial No. 129,128

14 Claims.- 1

This invention relates to a chromiumrplatin bathhaving exceptional plating. performance and a selferegulated catalyst acid radical contentsto compositions for maitingup such baths .andto a chromium plating. process and'provides im-. provernents therein.

It. has heretofore been proposed. to provide chromium plating baths in which the. catalyst acid radical. content is self-maintained or regulated with relation .to a given chromicacid concentration, to provide a ratio of CF03 to catalyst acid radical which. gives satisfactory commercial chromimn plating. Particular refcrenceismad toth' Lulrens Patent No. 10423611. A sparingly soluble sulphate compound. (SrSGn is known from said patent, will give: CrOs= M1504 ratios of approximately 1(i0to: 1 with soluti-ons of about .200'g./1..Chr0mic acid. Mostacommercial chromium plating solutions have a .chromic acid content higher than 2.98 g./1.,. commonly upto 490' to Edda/1. No means hase' been heretofore discovered of providing for the. self-regulation of the catalyst-acid radical contentof commercially usable bath solutions containing the higher concentrations of chromic acid, such. as 250 g-./1.' to 5G!) g./l.

According to the present invention, an aqueous chromium plating bath (anda composition .for making up such a bath) is provided, which has a catalyst acid radical content supplied from strontium sulphate and potassium ,silicofluoride, these two compounds .heimg presentin excess';;of their solubilities in theqbath, and Which- ,hasa wide range of .ChI-OllliC acid concentrations, said bath giving superior performancaandbeing selfregulating as to the catalyst acid radical content insolution.

lhis invention also provides a chromium plating process, using modes of procedure in baths according to the present-inventionfor attaining (1-) fast deposition of thick, good quality bright chromium plate, (2) fast deposition of thick, good quality gray chromium plate and (3') high covering power in the deposition of bright chromium plate in usual-decorativeplate thicknesses.

The bath (aqueous) essentially consists of about 290 to 500 g/l. Cl'Oa, strontium sulphate (SI'SOU present in. excess of its solubilityin the chromic acid solution and potassium silicofluoride (KaSi-Fe) also present in'excess of itssolubility in the chromic acid solution.

The use of chromic acid of high purity is recommended, such as. chromic acidcontaining less than 0.1% by weight of sulphate'andno other catalyst acid radicals. Chromic acidzor higher sulphate content can be used by first and: lysing for S04 and then adding approximately the stoichiornetric equivalent of strontium carbonate to form SrSCM with the S94 impurity, or byadding the stoiehiometric equivalent of barium carbonate, barium hydroxide, or barium chro mate for the SO;- in the chromic acid.

Either a C. P. (chemically pure)" or a good: technical grade of strontium sulphate may be used. One form or strontium sulphate comm-ercially available the mineral-Celestite which may-be also used-satisfactorily.

Either C. P; (chemically pure) or' a good tech nioal grade of KzSiFe canhe-used.

The SrSO; and KzSiFs instead of being added as such to the composition of matter or to the bath, may be added in the form of non-catalytic strontium and potassiumeornpounds and S04 and SlFt compounds which will interact solution to form STSO-4 and KzSiFs in amountswhichwill more than saturatethesolutions and provide aniundissolved excess or reservoir of SrSOrand KzSiFc from which S04 and SiFscatalystacid radicals lostfrom the bathin the course of operation will bereplenished.

Examples of non-catalytic strontium and p0:- tassiumg compounds are 51 60.3, Sr(QH)i, SIC'1'O4, etc. andKQI-l, KzCQspKoGrOr, I2GI'207. Examples of. vsuitable S94- and .SiFc compounds --reierred toyabove EmeHzSQn NtzSOr, Lil-255.84, (NT-I4). 2304, QaSKDsetc. and .I-IzSiFs, NaSiF (NI-maSiFfs, NIgSlF-fs, CaSiEs;,. etc

A solid composition of mattermaybe-therefore preparedoomprisin-g. C1 03. SrSOe'and KaSiFe as such, orby adding CrOa, and a suitable strontium compound. a suitable. sulphate compound, asuitable potassium compound and ;a suitablesilicoi'iuoridecompoundjas sug ested above. 011 add. ing both. types offcomposition of matter to water to give CrOa concentrations Within-the range 200 to 500 g./I.-,,,the end-result will he the same.

The. composition consistsoi chromic acid and enough SrSOiand .KzS'iE'c to provide as aminimum an undissolved excess of 81 504 andKzSiFt at any. desired bath concentration within the CrO's range of concentration, and at any desired temperature. within the operating range. of temperature.

Examples ofcompositions-are as-follows:

Example I I N Bartsby weight Cr03,9.'7.7%..

see s. 0.6%--- 0:61 Ks iFs, I.7'%.-.. 6....--. 1.74

3 The above composition will provide a bath solution saturated at 90 F. with SrSO4 and KzSiFs using this composition of matter in water to give CrOa concentrations of or near 500 g./l.

Example H Parts by weight CrOz, 95.3% 100 SrSOi, 1.4% 1.47 KzSiFs, 3.3% 3.5

The above composition will provide a bath solution saturated at 90 F. with SrSO4 and K2SiFs at minimum CIO3 concentration (200 g./l.), and an excess of SrSO4 and KzSiFs at higher CrOs concentrations, using this composition of matter.

Example III Parts by weight CrO3, 90.1% 100 SrSO4, 2.7% 3 KzSiFe, 7.2% 8

- in initial bath makeup, and to insure always having present an undissolved excess. Obviously there is no theoretical upper limit to the quantitles of SrSO4 and K2SiF6 that can be used.

Example IV Parts by weight CrOs, 92.5% 100 SrSO4, 2.0% 2.16 KzSiFs, 5.5% 5.95

A single composition, as Example III, has the advantage of having universal suitability throughout the full range of C1O3 concentration, but the composition such as Example II having lesser percentages of SrSOa and KzSiFe also are suitable for bath solutions of higher CrOa content. Example I represents the minimum percentages of SrSO-r and KzsiFs usuable at the maximum CIOs concentration and hence the minimum percentage which can be used and still be within the range of CrO3 bath concentrations herein disclosed. Example IV represents a composition in commercial use.

This bath makeup has a much higher degree of flexibility than any previously known chromium plating bath as regards the relationship of the catalyst acid radicals to the chromic acid content, and in addition the bath is self-regulating.

There is a great advantage in the self-regulating feature of the present bath by its obviating need for analytical determination of silicofluoride content, in that analyses for silicofiuoride in chromic acid are very difficult to make.

The efiect of the silicofluoride in the bath does not appear to be in arithmetic progression (additive) in relation to quantity added; that is, the catalytic effect of the SiF's in the present bath was found to have no definite or predictable effect by analogy to CrO3/SO4 current efficiency curves. The effect of the silicofluoride also appears to be affected by the amount of S04 present, the amount of CIOs and the temperature of the bath.

Chromium plating baths containing silicofluoride went into considerable use some fifteen years ago, but have largely gone out of use, ,due probably to the difficulty of analysing them, the difficulty of maintaining uniform operating characteristics, and to the fact that deposits other than very thin deposits were pimply. The deposits obtained by the use of the present inven tion are not pimply. The deposits are smooth at all 0103 concentrations, have little or no tendency to tree at high plating speeds and have leveling action. Deposits over about one-half thousandth of an inch from baths containing from 400 to 500 g./l. CIOs have a matte appearance, and are exceptionally smooth and free from nodules. Deposits in decorative plate thicknesses are bright throughout the CrOa concentration range.

The performance of baths according to the present invention is exceptional, and not in accordance with what had been expected from conventional ideas as to the efiect of the CrOa/total catalyst acid radical ratio.

In contrast with the generally used Grog/S04 chromium plating bath, a bath according to the present invention containing SrSO; and KzSiFo present in excess of the solubilities thereof in the bath can vary over a wide range of CrOa concentrations, ranging from about 200 g./l. to 500 g./ with neither a wide nor a signficant variation in the current efficiencies at commonly used temperatures over a wide range of current densities (1-6 amperes per square inch at 130 F. for instance). Moreover, the current efliciences of baths within the present invention are generally higher than the best current efiiciencies obtainable with the CrO3/SO4 baths; and, within the CIO3 range, and, at separate parts of the CI'O: range, are found baths having superior performance in differing particulars. Baths having optimum throwing power are obtained with a. CrOa content of about 450 g./l., with excellent throwing power at 400 to 500 g./l. ClOa. For barrel plating a 450 g./l. CIOa concentration has been found to give optimum performance.

Baths containing from about 250 g./l. to 350 g./l. CrOs, with optimum at about 325 g./l. CrOa, have been found to give the highest current efliciencies and highest plating speeds, and bright deposits in heavy thicknesses. The 250 to 350 g./l. CrOa range is recommended for so called hard chromium plating, where plating speed is of great importance and bright deposits in heavy thickness are desired. High plating speed results from high current efiiciency, and the feasible use of higher current densities. As compared with the conventional Grog/S04 bath, the

bath of the present invention within the 250 to 350 ClOs range (optimum 325 g./l. C1O3) has the advantage of considerably higher current efficiency and less tendency for treeing and has leveling action.

The current efficiencies of aqueous baths containing 200 g./l. to 500 g./l. CrOs saturated with SrSO4 and KzSiFs at 130 F. (54.4 0.), 110 F. (433 C.) and F. (322 C.) for representative ranges of current densities are shown in the graphs, Figs. 1, 2 and 3. Current efficiencies for baths containing more than 350 g./l. CrO3 are still excellent; but above 350 g./l. the deposits in thicknesses above about 0.0001 range, in appearance, from satiny semi-bright to full dull gray matte and, in texture, remain unusually smooth and velvety. Although bright deposits are generally preferred for so called hard or industrial chromium plate, this gray matte deposit is an excellent plate, and, as with the definite leveling action of surface scratches or almanac ablasior platingzbrightgdirectly on stainlessstesl and: r'zinc 'base. idiewcastings.

Another xadrantage. of then present 1 baths. is that if abrightnickel suniacais .passivc=,:or.=otherwiser not-receptive to a.- goodv chromium deposit, or: the, current; density is-too .low tor; complete: coverage, an. irrid'escent oxide, or oxidbschromatc filmicom-monly obtainedwith the conventional CrOa/SOi bath not. be; left onwthe :areassnot catered. by: the chromium Atpartly chromium covered inickeleplatedrpart plated :im the present bathcan have the chromium .stripped rrom. it and be replated with chromium without .th'a necessity of. :ro-bufiingzr ths nickel beiorereplatingv with. Azpartly chromium covered. nickel plated part plated inthe. ordinary chromic acid-sulphate bath. would have an. irridescent oxideior oxidc-chromate film. on. the uncovered. parts, and the. parts covered: by the .fi-lm would be etchedrin; stripping thc-chromium, and re-bufling would be .nwluiredr,beforev ire-plating. with" chi-o-v mium:

The present. baths. glaze. chromium deposits which. arev less 'sensitivezto interruption of current. which in. the chromica acidi-zsu lphats.bath. ordinarily causes: objectionable .grayydeposits. or poor-adhesion; and havenza betteractivating ace: tion on .ni'clrel plate than. thsusuali chromi'c. acidsulphate bath.

The: process for the. tast depositiomoi thick, good: quality; bright: chromium plate .is: carried out. .as follows: The: plating bath .is one according' to: this; invention. having: a. vCrOa concentration of. 200110 350 g./l.- (3:25 g.,/l optimum-) Thick; bright deposits... (ll-.0011 or more by way r of example) may, be electroplated at. current: densities and bath temperatures;generallyused for chromium deposition (see: ModernElectroplating, The Electrochemical Society; 1:942}. chapter on Chromium Plating, and particularly the charts on pages: 122; and 123). Thchigher temperatures and: current densities; are usually employed for obtaining thick deposits; andrsuchare bestused. in thcprescnt. process for rapidly obtaining thick deposits, the advantage here being that current emci'encies better than are obtained inthe conventional CrOz/S'Os bath are obtained, and higher (0.001 or 'moreby way-of example) magi-tbs electroplated at current densities. and zbathctemq peratures; generally used, for: chromium deposition (see Modern: Electroplating; referred: to above) The higher temperatures and current. densitiesarousually employed iorobtainingthick deposits; and :such' are .best uscdsin'the preseut process. for rapidly, obtaining thick :deposits; the advantage here being that the'current efficiencies are betterwthan those obtainedhy the-conventional crOs/socbaths and higher current densities canbe used without. a rough or trecdf plate being obtained. In fact the. thick gray-chromium deposits are especially smooth and free-+- from nodules. Where chromium plate must be ground or other-wise finished for dimensionalsrequirements or. fit, deposits such: as are produced by; the present process are particularly advantageous. Figure 1 shows the-relativelyhigh currentefil-v ciencies; which can .be' obtained at 1'-6 amperes per square inch, temperature. 130 (about .55" C.),, particularly in a. bath. with 1a...CrOc-conccn.-.

' tration of about 350 to 500 g./l.

The process for attaining high covering power in the deposition of bright chromium platc in usual decorative plate, is carried outas-follows: The plating bath isoneaccording to. the inven: tion, having a. CrOa concentration of. SEW-to 5'00.g./l. (400 to .500'g./l optimum) Theldcposits are the kind used for .soicalled decorativebright plate and commonly are a. iew:hundredth'thou.-; sandths inch thick. These .decorativebright-idw posits. may be elcctroplatedsat current densities and bath temperatures generally used ."fOIfiChI'O mium deposition (see: Modern Elcctroplatin'g, referred to above). Speedof platingusuall-ygnot. being as important for decorative-plate as'i'for thick so called hard chromium.plate; .a.-consider-'- able choice of temperatures and currentdensities is available; Figures 1,. 2- and 3-showexamples of temperatures and current densities zior The curves for the lowest currentpdensitiesein these three figures are indicative-of. the better covering power of the solutionslat thahigher chromic acid concentrations. The. chromium plate obtained is unusually bright;

The .processfor attaining a bright deposit in achromium plating barrel is carried. out. as; :follows:

The plating bathis: one accordingpto themsventio-n having a CrO-sxconcentrationvof. 4601' as optimum. The bright deposits .arecle'ctrw plated at current densities andbathtemperatnres generally used for chromiunutdeposition' in barrels. A temperature in the neighborhoodi ofi F. is often usedfor barrel plating. tcmperature and current density ofitherbatn, :to-zsoma extent, dependson thefiwork: being platedtfithe temperature: and: current density being always; fixed at those that give: the best covcragez This application is a continuation in part-oi application Serial N0 69,534;- fi led-Janu'aty'fig 1949:, now: abandoned.

What is claimed is:

I. A. composition or matter for making'up anaqueous chromium plating: bath comprising-essentiall-y' chromic acid-1' strontiumsulplrats and potassium sili'cofiuori de; there-"beingaboutwparts by weight of strontium sulphate and" about 8*- parts by weight potassium silicofluoride to'zl'fiil parts-by weight chromi'c acidi said composition when dissolved in waterbeing, adaptedto produce: a solution lia-ving a concentration. of 200' to' 250'0' gr/f. chromic'acid.

2. A composition of matter for making up an aqueous chromium plating bath, comprising essentially chromic acid, strontium sulphate and potassium silicofiuoride, there being about 2.16 parts by weight strontium sulphate and about 5.95 parts'by weight potassium s'ilicofiuoride to 100 parts by weight chromic acid, said composition when dissolved in water being adapted to produce a. solution having a concentration of 200 to 500 g./l. chromic acid,

3. An aqueous bath for chromium plating consisting essentially of 200 to 500 -g./l, chromic acid, and strontium sulphate and potassium silicofi-uoride present each in amounts in excess of the solubility thereof in the chromic acid solution.

4. An aqueous bath for chromium plating consisting essentially of 400 to 500 g./l. chromic acid, and strontium sulphate and potassium silicofluoride present each in amounts in excess of the solubility thereof in the chromic acid solution.

5. An aqueous bath for chromium plating consisting essentially of 250 to 350 g./l. chromic acid. and strontium sulphate and potassium silicoiluoride present each in amounts in excess of the solubility thereof in the chromic acid solution.

6. 'A process of electrodepositing chromium consisting essentially of passing current from an anode to an article-cathode immersed in a bath consisting essentially of 200 to 500 g./l. chromic acid and strontium sulphate and potassium silicofluoride present in the bath each in excess of its solubility therein.

7. A process of electrodepositing thick, bright chromium of good quality consisting essentially of passing current from an anode to an articlec-athode immersed in a bath consisting essentially of 200 to 350 g./l. C103 and strontium sulphate and potassium silicofluoride present in the bath each in excess of its solubility therein.

8. A process of electroclepositing chromium of good quality consisting essentially of 'passing'current from an anode to an article-cathode immersed in a bath consisting essentially of 350 to 500g./l. Ci'Os and strontium sulphate and potassium silicofiuoride present in the bath each in excess of its solubility therein.

9. A process of electrodepositing bright chromium in plating barrels, consisting essentially of passing current from an anode to an articlecathode immersed in a bath consisting of 450 gQ/l. ClOs and strontium sulphate and potassium sili'cofluoride present in the bath each in excess of its solubility therein.

10; A composition of matter for making up and maintaining aqueous baths for chromium plating comprising essentially chromic acid, strontium sulphate, and potassium silicofluoride, the strontium sulphate content being in excess of about 0.61 part by weight to 100 parts by weight of chromic acid and the potassium silicofiuoride content being in excess of about 1.74 parts by weight to 100 parts by weight of chromic acid, said composition when dissolved in water to produce a solution having 200 to 500 g./l. of chromic acid dissolved therein, also producing in such solution dissolved amounts of strontium sulphate and potassium silicofluoride suificient to saturate said solution, and also undissolved amounts of strontium sulphate and potassium silicofluoride.

11. A composition of matter for making up and maintaining aqueous baths for chromium plating comprising essentially chromic acid, strontium sulphate, and potassium silicofiuoride, the

strontium sulphate content being in excess of about 0.61 part by weight to parts by weight of chromic acid and the potassium silicofiuoride content being in excess of about 1.74 parts by weight to 100 parts by weight of chromic acid, said composition when dissolved in water being adapted to produce a solution having excess undissolved amounts of strontium sulphate and potassium silicofluoride.

12. A composition of matter for making up and maintaining aqueous baths for chromium plating comprising essentially chromic acid, strontium sulphate, and potassium silicofluoride. the strontium sulphate content being in excess of about 0.61 part by weight to 100 parts by weight of chromic acid and the potassium silicofiuoride content being in excess of about 1.74 parts by weight to 100 parts by weight of chromic acid.

13. A composition of matter for making up and maintaining an aqueous chromium plating bath comprising essentially chromic acid, strontium sulphate and potassium silicofluoride. there being at least about 1.47 parts by weight of strontium sulphate and at least about 3.5 parts by weight of potassium silicofiuoride to 100 parts by weight 01' chromic acid, said composition when dissolved in water being adapted to produce a solution having a concentration of 200 to 500 g./l. o1 chromic acid.

14. A composition of matter for making up and maintaining an aqueous chromium plating bath comprising essentially chromic acid, (1) a bathsoluble compound containing catalytic sulphate radical, (2) a bath-soluble strontium compound of a non-catalytic anion, the amount of said strontium compound being stoichiometrically equivalent to the amount of said sulphate compound so as to react therewith in said bath to form strontium sulphate, (3) a bath-soluble compound containing catalytic silicofiuoride radical, and (4) a bath-soluble potassium compound of a non-catalytic anion, the amount of said potassium compound being stoichiometrically equivalent to the amount of said silicofluoride compound so as to react therewith in said bath to form potassium silicofluoride, the said four compounds being present in such quantities to form at least about 0.61 part by weight strontium sulphate to 100 parts by weight of chromic acid and at least about 1.74 parts by weight of potassium silicofluoride to 100 parts by weight 01 chromic acid, said composition when dissolved in water to produce a solution having 200 to 500 g./l. of chromic acid dissolved therein, also producing in such solution dissolved amounts of strontium sulphate and potassium silicofluoride suiiicient to saturate said solution, and also undissolved amounts of strontium sulphate and potassium silicofluoride.

FRANK PASSAL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,815,081 Sohn et al. July 21, 1931 1,844,751 Fink et al. Feb. 9, 1932 1,864,014 Ewing June 21. 1932 1,928,284 Fink et a1. Sept. 26, 1933 2,042,611 Lukens June 2, 1936 FOREIGN PATENTS Number Country Date 566,218 Great Britain Dec. 19, 1944 

1. A COMPOSITION OF MATTER FOR MAKING UP AN AQUEOUS CHROMIUM PLATING BATH COMPRISING ESSENTIALLY CHROMIC ACID, STRONTIUM SULPHATE AND POTASSIUM SILICOFLUORIDE, THERE BEING ABOUT 3 PARTS BY WEIGHT OF STRONTIUM SULPHATE AND ABOUT 8 PARTS BY WEIGHT POTASSIUM SILICOFLUORIDE TO 100 PARTS BY WEIGHT CHROMIC ACID, SAID COMPOSITION WHEN DISSOLVED IN WATER BEING ADAPTED TO PRODUCE A SOLUTION HAVING A CONCENTRATION OF 200 TO 500 G./1. CHROMIC ACID. 